1. Field of the Invention
The invention relates to an energy storage device, and more particularly to an energy storage device with a negative electrode covered by a protective layer.
2. Description of the Related Art
With development of portable electronic products and mobile devices, an energy storage device with high energy, rapid charge/discharge and long-term use is desirable.
Generally, energy storage devices are divided into batteries and capacitors.
For batteries, electric energy is stored through chemical redox, in accordance with long-term small current discharge. Thus, high-energy-density electrode materials capable of long-term use are required. However, the redox is slow. For example, charging for a couple of hours is required, thus making it not suitable for use in high-power output. Additionally, if the batteries are unwillingly operated for delivering high-power output, battery quality will deteriorate.
For capacitors, electric energy is stored through physical charge accumulation. Charge/discharge is rapidly achieved. For example, a high-power supercapacitor is rapidly charged/discharged and provides high current in a very short time. However, their electric energy is stored through physical adsorption/desorption between an electrode and electrolyte-ion, resulting in low energy storage, and is merely applied in emergency power supply.
Thus, development of an energy storage device with high energy and high power is desirable.
In devices, stored energy (E) is proportional to capacity (Q) and working voltage (V) of electrode. A supercapacitor is composed of two high-surface-area carbon or metal oxide symmetric electrodes and aqueous or non-aqueous electrolyte containing ion salt. However, its working voltage merely achieves 1.2V in aqueous electrolyte and 2.5-2.7V in non-aqueous electrolyte, respectively. Also, metal oxides with redox capability such as RuO2 are expensive. To improve applicable potential window and working voltage, various devices with asymmetric electrodes have been developed. That is, positive and negative electrodes are composed of different materials have been developed.
U.S. Pat. No. 5,953,204 discloses an electric double-layer capacitor comprising an activated carbon positive electrode and a negative electrode, with an operation voltage of 4.0V. The negative electrode comprises a porous nickel current collector, without formation of alloy with lithium ion, with a carbon material capable of lithium ion intercalation/deintercalation therebetween formed thereon. WO 2005/096333 discloses an organic electrolyte capacitor comprising an activated carbon positive electrode and a mesoporous carbon negative electrode capable of lithium ion intercalation therebetween, with a working voltage of 4.0V. However, lithium is easily deposited on a carbon surface during rapid charge/discharge and penetrates a separator to result in short circuiting due to the intercalation reaction potential between the carbon and lithium ion when nearing 0V. Additionally, to achieve high capacity, the foregoing carbon negative electrode with a specific structure is required, thus chemical fabrication is complex and expensive. U.S. Pat. No. 6,252,762 discloses a capacitor comprising an activated carbon positive electrode and a Li4Ti5O12 negative electrode capable of lithium ion intercalation therebetween, with a working voltage of 3.0V. However, the reaction potential between Li4Ti5O12 and lithium ion is about 1.5V higher than that between carbon and lithium ion, resulting in a narrow applicable potential window. Also, the material shows good capacity retention only under a specific synthesis condition that is complicated and hard to control.
Among the foregoing negative electrode materials, the theoretical capacity of carbon is 372 mAh/g, however, lithium is easily deposited on a carbon surface during rapid charge/discharge and penetrates a separator to result in short circuiting due to the intercalation reaction potential between the carbon and lithium ion when nearing 0V. The reaction potential between Li4Ti5O12 and lithium ion is about 1.5V higher than that between carbon and lithium ion. Although safety is promoted, an applicable potential window is narrowed. Additionally, a great quantity of Li4Ti5O12 is required to prepare a large capacity device due to its low capacity of 140-160 mAh/g, increasing device weight.
Additionally, some metal materials, for example, antimony, bismuth, silicon, tin, lead, aluminum, gallium, indium, cadmium or zinc capable of reaction with lithium ion to form alloy are also used as a negative electrode, wherein they have a higher capacity than carbon. However, when lithium ion is intercalated into such negative electrode materials to form an alloy, the metal material is expanded, and when lithium ion is deintercalated from the alloy, it is contracted to its original size. Stresses generated from violent volume expansions and contractions during intercalation/deintercalation causes material cracks, reducing device lifespan. Thus, when such alloy negative electrodes are utilized, stability of the electrode structure should be considered.